The present invention relates to the process control industry. In particular, the invention relates to a pressure sensor in a pressure transmitter.
Pressure transmitters in process applications measure pressure of a process and responsively communicate the information over a two-wire process application loop, for example a 4-20 mA current loop. Pressure sensors in transmitters typically comprise some type of a pressure responsive structure which has a deflectable diaphragm that moves in response to applied pressure. These structures can be used to measure both absolute and differential pressure. As used herein, a differential pressure sensor is a sensor which measures a relatively small pressure differential (such as that generated across an orifice in a flow tube or between two different heights in a fluid filled container) over a relatively wide absolute pressure range. In a typical prior art transmitter, to measure differential pressure, two different pressures are applied to opposing sides of the structure causing a relative deformation in the structure which is measured. Measurement of the deformation, for example, can be achieved by measuring a change in electrical capacitance due to movement of capacitor plates carried on the structure, by change in resistance of a resistive strain gauge, etc.
Highly accurate absolute pressure sensors have been desired. It also may be preferable desirable to measure differential pressure with two separate absolute pressure sensors, employing two separate absolute pressure sensors is mechanically much simpler than mechanically coupling two pressures to a differential pressure sensor. Additionally, an over-pressure condition in such a differential pressure sensor can damage the differential pressure sensor. However, it has been difficult to obtain absolute pressure sensors with sufficient accuracy to allow differential pressures in the 0.4 psi to 40 psi range to be measured in a device which must withstand static or line pressure extremes of as much as 4000 psia. For example, 0.01% of 4 psid requires 0.00001% of 4000 psia (10−7 or 0.1 ppm).
Typical known pressure sensors used in process applications have unit-to-unit variations in sensitivity to sensed pressure as well as unit-to-unit variations in undesired responses to extraneous parameters such as temperature. This can be a particular problem when the outputs of two absolute or gauge pressure sensors are combined to provide an output representing differential pressure or when the sensor is used over a large pressure range. Additionally, mechanical stress associated with mounting the sensor to the pressure transmitter may result in relatively large errors in pressure measurement.
Another type of pressure sensor is described in U.S. Pat. No. 6,484,585 entitled PRESSURE SENSOR FOR A PRESSURE TRANSMITTER assigned to Rosemount Inc. by Sittler et al. which issued Nov. 26, 2002. The Sittler et al. patent describes a different type of pressure sensor which is made of a brittle material. Capacitive plates are carried in the material and spacing between the capacitive plates changes in response to an applied pressure. This causes the electrical capacitance between the plates to change which can be measured and correlated to the applied pressure.